1. Field of the Invention
This invention relates to an improved method and apparatus for reclaiming and thus recycling used abrasives that have been employed, for example, in surface preparation of steel objects prior to the application of paints or other protective coatings. More particularly, the novel method and apparatus provide for a fluidized bed treatment of the used abrasives with the assistance of suction, to remove contaminants and fines therefrom, thereby producing a high quality re-usable abrasive material well suited for subsequent blast cleaning operations.
2. Description of the Prior Art
Generally, the abrasive media falls into two categories, recyclable and expendable. Silica sand and slags are expendable, low cost abrasives and are, therefore, not considered recyclable. On the other hand, there are abrasives that do not easily break down at impact and are generally more expensive and worth recycling. Steelgrit, garnet, aluminum oxide, staurolite and hematite all belong to this group. Applicant's own U.S. Pat. No. 5,456,628 of Oct. 10, 1995 describes one such recyclable impact material.
The effect of the blast cleaning process on all abrasives resulting from high velocity impacting on steel surfaces being cleaned, causes them to abrade and generate contaminants such as dirt, paint, mill scale, and rust from the treated surface. At the same time a certain amount of fractured, fine abrasive particles are also produced. Generally, the used abrasive is considered reusable if the percentage of fines is not excessive and the contaminants can be extracted to a satisfactory level to meet health and environmental standards.
Various methods and equipment are employed in the abrasive blasting industry to allow spent abrasives to be reclaimed. Practically all presently employed abrasive recycling devices are operated on the principle that uses various air washing methods. In these methods, the relative densities of abrasives and contaminants are taken advantage of, because the abrasive is generally more dense than the contaminants. The contaminated abrasive is caused to fall along a vertical line, preferably in a continuous fashion, so that a uniform curtain of falling abrasive is obtained. An air stream is applied across the falling curtain with an objective to entrain the contaminants and the undesirable fractions of abrasive fines. Usually a cross-directional flow of the air stream is maintained so as to remove the undesirable part of the used abrasive in addition to the contaminants, and the decontaminated abrasive is then collected at the bottom of the cascading curtain for reuse. One such apparatus is described in U.S. Pat. No. 4,394,256 of Jul. 19, 1983.
In such a system, the uniformity of the falling curtain density is important because any opening or discontinuity in the curtain allows the air to accelerate. The resulting increased velocity at the fringe of such curtain openings or discontinuities also removes usable sizes of abrasive particles. On the other hand, reduced air velocity in the areas of denser curtain formation often results in incomplete washing. Such a system, therefore, has significant disadvantages.
Magnetic separation is also used to separate magnetically attractable abrasives from non-magnetically attractable contaminants. Such magnetic separation apparatus is disclosed, for example, in U.S. Pat. No. 5,657,876 of Aug. 19, 1997. This system can, however, be used only with magnetic materials, such as steel grit and shot, which excludes some 98% of the abrasives utilized by the industry.
There are also methods and apparatus using a combination of steps, including a physical separator, a rotary screen separator, a magnetic separator, and air wash separation and sub-atmospheric pressure, for example, as disclosed in U.S. Pat. No. 5,622,265 issued Apr. 22, 1997. Such systems are rather complex and expensive and thus have not achieved widespread acceptance in the industry.